Microporous polyolefin membranes are widely used for various applications such as separators for lithium batteries, etc., electrolytic capacitor membranes, various filters, moisture-permeable, waterproof clothes, reverse osmosis filtration membranes, ultrafiltration membranes, microfiltration membranes, etc.
To prevent the heat generation, ignition, explosion, etc. of batteries caused by the short-circuiting of external circuits, overcharge, etc., separators for lithium secondary batteries and lithium ion batteries are required to have not only a function to close fine pores when abnormal heat generation occurs, thereby stopping battery reactions, but also a function to keep their shapes even at high temperatures to prevent cathode materials and anode materials from coming into direct reaction. However, microporous polyethylene membranes produced by wet methods, which are widely used for separators at present, are vulnerable to oxidation when stored at high temperatures particularly in high-capacity batteries, resulting in lowered battery capacity, although they have high strength and low shutdown temperatures. On the other hand, separators formed by microporous polypropylene membranes produced by dry methods have low and largely variable strength, resulting in poor productivity of batteries, although they are less vulnerable to oxidation when stored at high temperatures.
As a porous film whose pores do not disappear in a wide temperature range with a high upper limit, JP7-304110 A proposes a laminated porous film obtained by alternately laminating a polypropylene film and a polyethylene film to three or more layers, and stretching the resultant laminate to make it porous.
As a microporous polyolefin membrane having excellent oxidation resistance, JP2003-92096 A proposes a microporous membrane containing 0.5% or less by mass of a polyolefin resin having a molecular weight of less than 10,000.
JP2003-59477 A proposes a battery comprising a laminated separator comprising one or more low-temperature-shrinkable microporous membranes, and one or more high-temperature-shrinkable microporous membranes having a higher endothermic temperature, which are laminated such that they are independently shrinkable. The battery of JP2003-59477 A has excellent safety because the shape of the high-temperature-shrinkable microporous membrane is maintained even at high temperatures.
However, because the laminated porous film of JP7-304110 A is produced by a dry method (method of making porous by stretching), it fails to overcome the problem of low productivity due to low and largely variable strength. In Examples of JP2003-92096 A, microporous membranes made only of polyethylene are produced, which have relatively low meltdown temperatures. Though JP2003-92096 A describes that polyethylene may be blended with polypropylene, the blend of polyethylene and polypropylene has elevated meltdown temperature and shutdown temperature, resulting in poor shutdown properties. Because the low-temperature-shrinkable membrane is vulnerable to oxidation when the separator of JP2003-59477 A is stored at high temperatures, the separator is not necessarily sufficient in a cycle life and a storing life at high temperatures.